As @acub points out, IE8 backward compatibility is always going to be a nightmare for web developers. Consider that, IE version 8 users use Windows XP since MS froze their IE development support for XP at IE 8. So the question is are you targeting your site at the ‘older population’ who are very reluctant to upgrade?? Bottom line, you need to know your audience and accept some design compromises.
I believe as long as the theme and your website does not fall apart, but degrade gracefully in the older browsers (is readable, etc.) — then I suggest you should be quite satisfied with Customizr outcome.
For myself, I ignore IE8 and below entirely. It is not worth the time and effort to make it “perfect”. I’d say you’re reaching the point of diminishing returns on your efforts. ??